1. Field of the Invention
This invention relates to switching regulators, and in particular to the stabilization of free-running switching regulators.
2. Description of the Prior Art
A typical free-running switching regulator such as that illustrated in FIG. 2 of U.S. Pat. No. 3,978,393, includes a transistor switch and an inductor which are connected between an unregulated DC source and a load. A filter capacitor is generally connected across output terminals of the regulator placing it in parallel with the load. The transistor switch is toggled at a relatively high rate (e.g. 10-20 KHz) by a voltage comparator which compares the load voltage to a desired regulated voltage of predetermined magnitude. The comparator maintains the load voltage at the regulated magnitude by controlling the duty cycle of the switch. For example, if the load voltage tends to rise, the duty cycle of the switch is decreased so that less current is delivered to the load during each cycle.
By using such switching regulators to supply power to loads voltage regulation can be effected over a wide range of loads at high efficiency. This high efficiency results because the transistor switch is either fully on or off, except for short transition periods between these two states, and relatively little power is dissipated thereby. Unfortunately, in conventional regulators of this type, stability of the switching frequency and of the feedback loop by which the comparator monitors the load voltage are dependent on the load impedance which typically varies widely. The resulting instabilities can best be understood by separately considering the effects of changes in the resistive and the reactive components of the load impedance.
The resistive component, hereinafter referred to as load resistance, includes the resistance of the load itself, the resistance of wiring connecting the load to the regulator and an equivalent series resistance (ESR) of the filter capacitor which is effectively in parallel with the former two resistances. During constant load operation the transistor switch is alternately closed (turned on) and opened (turned off) at a constant switching frequency. While the switch is closed the voltage differential across the inductor causes the current passing through the load resistance to increase at a constant rate (di/dt).sub.ON from a magnitude slightly below the DC level of the load current to a magnitude slightly above the DC level, where the voltage developed across the load resistance reaches a predetermined upper limit causing the comparator to open the switch. While the switch is open the voltage differential across the inductor causes the current passing through the load resistance to decrease at a constant rate (di/dt).sub.OFF to the magnitude slightly below the DC level, where the voltage developed across the load resistance reaches a predetermined lower limit causing the comparator to again close the switch.
The rate of change of the voltage developed across the load resistance is dependent on both the rate of change of the current passing therethrough and on the magnitude of the load resistance itself. As long as this resistance remains constant, the rate of change of this voltage also remains constant and the comparator continues to toggle the switch at a constant rate. If the load resistance changes, however, the corresponding rate of change of the load voltage causes the switching frequency to change. Such changes in load resistance are caused not only by changes in the resistance of the load itself, but also by variations in the ESR of the filter capacitor. The ESR of aluminum electrolytic capacitors typically used for filter capacitors varies substantially with temperature.
Changes in switching frequency are undesirable because the regulator is designed to operate most efficiently at a nominal frequency. If this frequency is substantially exceeded the power dissipated by the switching transistor during the transitions between the on and off states increases causing inefficient regulator operation and possibly causing destruction of the transistor.
The reactive component of the load impedance, hereinafter referred to as load reactance, includes the reactance of the load itself, the filter capacitance and the lumped reactance of wiring connecting the load to the regulator. Because these reactances are in the feedback loop, they cause phase shifts of the feedback voltage. If the load reactance is increased to a magnitude which is sufficient to cause a substantial phase shift, feedback loop instability can occur causing the comparator to toggle the switch out of phase with variations in the load voltage.